This invention relates to a process for manufacturing a cold-rolled high strength steel sheet having excellent cold workability, and to the coldrolled high strength steel sheet manufactured by said process, and further to a car body comprising said cold-rolled high strength steel sheet. Such cold rolled steel sheet for car body has preferably a thickness of 3.0mm or less.
Recently constant efforts have been made in the fabrication of cars to reduce the weight of car body not only from the view points of meeting sefety requirements and reducing pollution by exhaust gases, but also as a means for reducing the amount of fuel consumed. For accomplishing these aims, cold-rolled high strength steel sheets have come into wide use in place of conventional mild steel sheets.
U.S. Pat. No. 3,830,699, issued to T. Matsuoka et al, discloses a process for manufacturing such steel sheet: The subject matter of the invention of this U.S. Patent resides in making a steel comprising 0.03 - 0.2%C, 1.6 - 3.0% Mn, 0.03 - 0.6% Si, 0.01 - 0.25% Nb and/or 0.01 - 0.2% Ti, and the remainder of Fe and inevitable impurities, hot rolling and cold rolling into a steel sheet of thickness of 3 mm or less, and annealing the steel sheet at a temperature of 620.degree. C to A.sub.3 transformation point. In the invention of this U.S. patent, the amount of Si and Mn contained are specified as claimed because silicon contained in excess of 0.6% increases the brittleness and deteriorates the weldability of the steel and manganese contained at less than 1.6% does not impart to the steel a tensile strength of 50 kg/mm.sup.2 or more.